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Number 385, August 3, 1998 by Phillip F. Schewe and Ben Stein
GROUP DECISIONS CAN BE MATHEMATICALLY UNPREDICTABLE even with total knowledge of everyone's individual choices and the use of completely explicit decision making rules, a new study shows. As it turns out, the order in which the choices are presented to a group can make the course of the decision process impossible to anticipate. Not only does this new result address the murky human process of amending Congressional bills, but it also confirms the idea that groups of computerized "intelligent agents" each with its own rules for buying and selling commodities can cause prices to fluctuate in a mathematically chaotic fashion. Whenever a group tries to choose among three or more options by weighing two at a time, its decision can often cycle endlessly from one choice to another, especially when there is a large diversity of preferences. To address the consequences of such cycles, David Meyer (UC-San Diego, 619-534-5524) and Thad Brown (Univ. Missouri) model each possible sequence of choices as a minimum-energy configuration of a one-dimensional system, such as the string of atoms with up and down spins used in studying magnetic materials. For those sequences that cycle endlessly through different options, the system has a greater-than- zero entropy--meaning that there is an uncertainty in what the group's next choice will be. In this case the dynamics are chaotic---the next choice in the cycle is unpredictable since it depends sensitively on the order in which options are presented to the group. (Meyer and Brown, Physical Review Letters, 24 August 1998.) See figure at Physics News Graphics
THE FIRST OBSERVATION OF LASER WAKEFIELD ACCELERATION OF INJECTED ELECTRONS has been announced by a team of French scientists. Their research is part of an effort to develop a more compact method for accelerating particles to high energies using the strong electric fields in laser light. In the wakefield approach a short laser pulse is sent through a plasma, exciting waves of positive-charge and negative-charge regions. An injected particle starting with just the right velocity can ride these plasma waves (which spread out like the wake behind a boat) to ever higher speeds. In the French experiment, maximum longitudinal electric fields of 1.5 GV/m were achieved. The electron energy gain was as high as 1.6 MeV. (F. Amiranoff et al., Physical Review Letters, 3 August 1998; background article in Scientific American, March 1989.)
NO END IN SIGHT FOR COSMIC RAY ENERGIES. Putting terrestrial accelerators to shame, nature has contrived to imbue some particles with energies greater than 1020 electron volts. But these high-end cosmic ray events---only a mere handful have been recorded so far--- would seem to be at odds with the idea that interactions with the cosmic microwave background act as a sort of universal brake, permitting energies not much above 1019.6 eV (the so called Griesen-Zasepin-Kuz'min, or GZK, limit). It didn't help that for some time there was a relative scarcity of events in the energy range between 1019.6 and 1020 eV. But new data reported by the Akeno Giant Air Shower Array (AGASA) collaboration in Japan (Masahiro Takeda et al., mtakeda@icrr.u-tokyo.ac.jp) fills in this gap, strengthening the statistical argument that either the GZK cutoff is not working as planned or that some unexpected process is producing the highest-energy rays.In other words, there seems to be no limit to cosmic ray energy. (Takeda et al., Physical Review Letters, 10 August 1998.)
PHYSICS NEWS UPDATE now goes on a summertime recess of three weeks.
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